1. (Field of the Invention)
The present invention relates to an element for display
device and more specifically to a high quality EL element comprising an organic thin film.
2. (Description of the Prior Art)
There has increasingly been desired for the development of high quality parts for displaying information along with the recent remarkable development of the information-oriented society. Electroluminescence elements (EL elements) which make use of the phenomenon of electroluminescence (EL) have been attracting much attention from the industrial standpoint because of their excellent properties such as visibility.
There have been developed, for instance, so-called intrinsic EL elements such as elements comprising organic substances in which inorganic fine particles are dispersed and elements in which inorganic thin films of, for instance, ZnS are sandwiched between insulating thin films and they have already been put into practical use. However, these elements suffer from various problems. For instance, they require high driving voltages and it is difficult to obtain elements capable of emitting lights of certain colors such as lights of blue color. On the other hand, there have also been known carrier injection EL elements in addition to these intrinsic EL elements. The carrier injection type EL element comprises electrons and holes injected in a p-n junction of, for instance, a semiconductor and emits light through recombination of the electrons and holes. The element is, for instance, characterized in that it can be operated by a direct current at a low driving voltage and that it can convert a current into a light in a high efficiency. In these elements, inorganic crystalline semiconductors such as GaP are mainly used. However, these elements likewise suffer from various problems. For instance, colors of emitted lights are limited to specific ones and it is difficult to obtain any element having a large area. There has accordingly been desired for the development of a technique for producing elements having a low driving voltage and capable of emitting light of various colors, which permits production of an element having a large area.
Recently a novel carrier injection type EL element which makes use of an organic thin film has been reported (see C.W. Tang, Appl. Phys. Lett., 1987, 51(12), p. 193) and has been attracted much attention since it is anticipated that colors of emitted lights can freely be selected through appropriate selection of organic compounds, that such element can be operated at a low driving voltage and that an element having a large area can easily be prepared by the application of method for forming thin films such as vapor-deposition and coating methods. Such EL elements produced from organic thin films are described in, for instance, U.S. Pat. Nos. 4,356,429; 4,539,507; 4,720,432; 4,769,292 and 4,885,211. In addition, the inventors of this invention developed an EL element comprising laminated organic and inorganic thin films (see Japanese Unexamined Patent Publication (hereinafter referred to as "J.P. KOKAI") Nos. Hei 2-196475 and Hei 2-207488). However, it has been reported that these EL elements comprising organic thin films suffer from so-called deterioration or reduction in luminance of emitted lights when they are operated over a long time period.
The foregoing element which makes use of the organic thin film has a layer structure comprising a hole-transport organic thin film and a emitted organic thin film formed between two electrodes. The first electrode mainly comprises ITO, while the second electrode comprises a metal thin film of, for instance, Mg. On the other hand, the element which comprises laminated organic and inorganic thin films has a layer structure comprising a hole-transport inorganic thin film and a luminous organic thin film formed between two electrodes. In this case, the first electrode likewise mainly comprises ITO, while the second electrode comprises a metal thin film of, for instance, Mg. The following deterioration phenomena are, for instance, observed in these elements: (i) if they are allowed to stand in the air, the driving voltage thereof increases and as a result, the elements are broken down; (ii) there are Observed an increase of the driving voltage and formation of dark spots which do not have capability of emission of light during the continuous operation thereof; and (iii) when increasing the current load for operation, the luminance thereof is substantially reduced due to generation of heat.